runtime: catch concurrent stacks more often
If two goroutines are racing on a map, one of them will exit
cleanly, clearing the hashWriting bit, and the other will
likely notice and panic. If we use XOR instead of OR to
set the bit in the first place, even numbers of racers will
hopefully all see the bit cleared and panic simultaneously,
giving the full set of available stacks. If a third racer
sneaks in, we are no worse than the current code, and
the generated code should be no more expensive.
In practice, this catches most racing goroutines even in
very tight races. See the demonstration program posted
on https://github.com/golang/go/issues/26703 for an example.
Fixes #26703
Change-Id: Idad17841a3127c24bd0a659b754734f70e307434
Reviewed-on: https://go-review.googlesource.com/126936
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
diff --git a/src/runtime/map.go b/src/runtime/map.go
index c03e745..c3fcfbf 100644
--- a/src/runtime/map.go
+++ b/src/runtime/map.go
@@ -567,7 +567,7 @@
// Set hashWriting after calling alg.hash, since alg.hash may panic,
// in which case we have not actually done a write.
- h.flags |= hashWriting
+ h.flags ^= hashWriting
if h.buckets == nil {
h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -679,7 +679,7 @@
// Set hashWriting after calling alg.hash, since alg.hash may panic,
// in which case we have not actually done a write (delete).
- h.flags |= hashWriting
+ h.flags ^= hashWriting
bucket := hash & bucketMask(h.B)
if h.growing() {
@@ -921,7 +921,7 @@
throw("concurrent map writes")
}
- h.flags |= hashWriting
+ h.flags ^= hashWriting
h.flags &^= sameSizeGrow
h.oldbuckets = nil
diff --git a/src/runtime/map_fast32.go b/src/runtime/map_fast32.go
index bf0b236..6715585 100644
--- a/src/runtime/map_fast32.go
+++ b/src/runtime/map_fast32.go
@@ -103,7 +103,7 @@
hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
// Set hashWriting after calling alg.hash for consistency with mapassign.
- h.flags |= hashWriting
+ h.flags ^= hashWriting
if h.buckets == nil {
h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -189,7 +189,7 @@
hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
// Set hashWriting after calling alg.hash for consistency with mapassign.
- h.flags |= hashWriting
+ h.flags ^= hashWriting
if h.buckets == nil {
h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -276,7 +276,7 @@
hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
// Set hashWriting after calling alg.hash for consistency with mapdelete
- h.flags |= hashWriting
+ h.flags ^= hashWriting
bucket := hash & bucketMask(h.B)
if h.growing() {
diff --git a/src/runtime/map_fast64.go b/src/runtime/map_fast64.go
index 4bde9e2..164a4dd 100644
--- a/src/runtime/map_fast64.go
+++ b/src/runtime/map_fast64.go
@@ -103,7 +103,7 @@
hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
// Set hashWriting after calling alg.hash for consistency with mapassign.
- h.flags |= hashWriting
+ h.flags ^= hashWriting
if h.buckets == nil {
h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -189,7 +189,7 @@
hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
// Set hashWriting after calling alg.hash for consistency with mapassign.
- h.flags |= hashWriting
+ h.flags ^= hashWriting
if h.buckets == nil {
h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -276,7 +276,7 @@
hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
// Set hashWriting after calling alg.hash for consistency with mapdelete
- h.flags |= hashWriting
+ h.flags ^= hashWriting
bucket := hash & bucketMask(h.B)
if h.growing() {
diff --git a/src/runtime/map_faststr.go b/src/runtime/map_faststr.go
index 415bbff..bee62df 100644
--- a/src/runtime/map_faststr.go
+++ b/src/runtime/map_faststr.go
@@ -202,7 +202,7 @@
hash := t.key.alg.hash(noescape(unsafe.Pointer(&s)), uintptr(h.hash0))
// Set hashWriting after calling alg.hash for consistency with mapassign.
- h.flags |= hashWriting
+ h.flags ^= hashWriting
if h.buckets == nil {
h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -294,7 +294,7 @@
hash := t.key.alg.hash(noescape(unsafe.Pointer(&ky)), uintptr(h.hash0))
// Set hashWriting after calling alg.hash for consistency with mapdelete
- h.flags |= hashWriting
+ h.flags ^= hashWriting
bucket := hash & bucketMask(h.B)
if h.growing() {
